There are only a few differences in a Commander sized frame (4.25" barrel) and a full sized frame (5" barrel). The Commander frames have a 3/4" shorter spring shroud but it isn't necessary to cut a full sized frame down. Purely a looks thing. Inside the gun there are some required changes: a Commander recoil spring will be 2 lbs heavier and shorter. The spring guide and cap are also shorter. Naturally, the barrel, barrel bushing, and slide are shorter too.
Brownell's stocks the 18lb Commander recoil spring, spring cap, spring guide, and bushing if you don't feel like chopping down existing parts.